Anionic acid dyeing of polyamide yarns involves the reaction of the amino end group of the nylon yarn with the sulfonic acid end group of the dye molecules.
Depending on their chemical structure, the anionic dyes could possess a mono-, or a di-, or a tri-sulfonic acid end group. The reactivity of the dye with the fiber is directly proportional to the number of functional groups present in the dye and/or the fiber. Therefore, it follows that the greater the number of dye molecules that bond with the amine endgroups of the fiber, the better the washfastness of the fiber.
Several applications involve treatment of heat to the fabric prior to dyeing. A typical example is the case of elastic fabrics which are knitted with elastomeric yarns, e.g. LYCRA.TM. (DuPont, Wilmington) which imparts stretch to the fabric. Heatsetting of the fabric prior to dyeing is essential to avoid curling of the fabric. Typical heat setting temperatures range between as low as 90.degree. C. to very severe temperatures of 200.degree. C. When heat setting is conducted at elevated temperatures such as above 140.degree. C., in air, oxidative degradation of the amino end groups occurs destroying the functional groups present in the fiber. This depletion of amino end groups reduces the affinity of the dye molecules to the fiber. Such a fabric picks up less dye than a non-heatset fabric, has a worse washfastness and has a dull appearance. In more severe cases the preheatset-and-dyed fabrics also exhibit streaky appearance. Therefore, there exists a need to improve the resistance to thermal degradation of polyamide yarns so as to retain the brightness of the fabric, improve the washfastness of the fabric and improve the uniformity of the dyed fabric.
To increase dye pick up of a heat set fabric, dyeing methods are modified. This involves increasing the temperature of the dye bath in some cases and/or reducing the pH of the dye bath, in many cases. Although, the modified dyeing procedure increases the affinity of the dye into the fiber, it is a temporary phenomenon, since after dyeing, the fabric is washed thoroughly to remove the acidity in the fabric. The dye molecules that are thus entrapped in the fiber, are loosely bound due to lack of chemically reactive sites in the fiber. Such molecules are susceptible to diffuse out of the fabric during subsequent washings. The physical size of these entrapped dye molecules have a significant influence the diffusion of the dye out of the fiber and hence, also the dye washfastness of the fabric. Thus fabrics dyed with smaller dye molecules would exhibit worse washfastness than larger ones. In many cases, the smaller dye molecules are also those which possess a mono-sulfonic acid group, i.e. the least number of functional groups, and hence a lesser affinity to the fiber. Pre-heatset polyamide yarns dyed with such dyes exhibit the worst washfastness.
To alleviate this problem, several methods have been invented. Most of these techniques involve a chemical treatment after the dyeing process. DE-A 4,131,926 describes a process wherein the dyed substrates like nylon are treated with dispersions of sterically hindered cycloaliphatic amines, which improves light and washfastness.
DE-A 3,330,120 discloses an aftertreatment of polyamide textiles, dyed with anionic dyes, with a polybasic compound which was a reaction product of a polymine with a cyanamide derivative to improve the wetfastness and washfastness.
Yet another method is disclosed in JP 81 53, 293 wherein acid dyed polyamide fibers are treated with a color fixing agent. This color fixing agent is based on a condensation product of a polysulfone, a compound containing amino groups and sulfonic acid groups, and an aldehyde. The washfastness of polyamide fibers treated with this agent is improved.
Similarly, JP 80 71,884 describes a polymeric quaternary ammonium compound which when applied to the face of a printed polyamide fabric, improves the colorfastness of the fabric.
Although improvements are claimed in washfastness by chemical aftertreatment processes, considerable deficiencies still exist in several applications. These relate to the fundamental issue of reduced affinity of certain dyes with the fiber due to the depletion of amino end groups during preprocessing of the polyamide fabric. A more important issue is that of increased cost of processing the fabric. The chemical aftertreatment not only involves the cost of an additional processing step but also the cost of chemical waste disposal and effluent water treatment. With tighter environment protection regulations on the types of disposable effluents, the economics of aftertreatments could get to be restrictive.
Therefore, there exists a need for a process that would improve the washfastness of polyamide yarns without increasing or altering the chemicals that are used currently in the dyebath. Furthermore, there also exists a need to achieve a better exhaustion of the dyebath so as to reduce the dyes and chemicals being released as effluents in the waste water.
U.S. Pat. No. 4,863,664 discloses a high speed process of making polyamide filaments by melt mixing polyamide with some additives like water, alcohols or organic acids prior to spinning. Although, the process claims to improve yarn quality, processability and dye washfastness of the fabric, it does not address the issue of heat stability of the fibers made from such a process. The poor heat stability and the resulting streaky dyeing are significant disadvantages of this process.
It was the object of the present invention to reduce or eliminate the deficiencies existing in current processes in relation to washfastness and heat stability of the fabric and provide a new process for the manufacture of polyamide fibers, with improved dye washfastness and heat stability.
In addition, it was also the object of the present invention to introduce a process for the manufacture of polyamide yarns which would possess reduced yellowing and retain the whiteness of the fabric after heat treatment.
Another object was to provide a process for the manufacture of polyamide fibers for the production of dyed fabrics having improved uniformity after heatsetting.
Yet another object was to provide a process for the manufacture of polyamide fibers which achieve a greater exhaustion of the dye bath at an increased rate thereby reducing the release of effluents of waste dyes and chemicals in waste water.
A further object was to provide a process for the manufacture of polyamide fibers for the production of dyed fabrics having deeper dye shades.
Since swimwear is one of the potential applications for the yarns of the present invention where resistance to fading in a chlorinated water pool is a major requirement, it was another objective of the present invention to provide a process for the manufacture of polyamide fibers which would possess improved resistance to color fading in a chlorinated water pool.